The significance of 90 A neurofilaments (NF) is not yet known and that of 240 A microtubules (MT) is only poorly understood. Recent advances now make it possible to study dynamic changes in the distribution and occurrence of each of these cytoplasmic structures. In addition their relation to intracellular chemical transport may now be examined. Normal developmental sequences of MT and NF will be studied in cells and fibers of the chick spinal cord. Reversible alterations in the number and distribution of NF and MT will be experimentally studied in thermal acclimation in poikilo- and hetero-therms and in degeneration and regeneration in the fish visual pathway. In each of these situations the approach will be similar: differential reduced silver procedures that specifically stain NF or NF plus MT will first be used for quantitative light microscopy. Electron micrography (EM) will then be used to study brain regions at the times of critical importance for comparing stable populations of NF and MT with those undergoing dynamic changes. Chemical transport will be investigated with high resolution EM radioautography using the "physical development" methods. Mosaic patterns of grains found over sectioned cells and fibers previously labelled with radioactive materials will be compared with those over adjacent serial sections and with intra-cellular distributions of NF, MT, tubular endoplasmic reticulum, and mitochondria. The results are expected to provide a basis for increased understanding of systems in which some knowledge of developmental, physiological and pathological changes already exists.